Control valves are used to regulate process fluid flow through a pipe or conduit. Such valves typically include a throttling element disposed in the process fluid flow path and connected to an actuator. While various types of actuators are known, many control valves use a pneumatic actuator which uses air, natural gas, or other fluid under pressure to adjust the position of the actuator. In a spring and diaphragm actuator, for example, a spring applies a force to one side of the actuator while fluid pressure is controlled on an opposite side of the actuator, thereby adjusting the position of the throttling element. Alternatively, a piston actuator may be used in which the piston divides the actuator housing into upper and lower chambers and the fluid pressures of both chambers are controlled to drive the actuator to a desired position. In any type of pneumatic actuator there may be a nominal bleed-off of the control fluid to atmosphere.
A positioner (or servo controller) controls the fluid pressure supplied to one or both chambers of a pneumatic actuator. The positioner typically includes a processor, a current to pressure (I/P) converter, second stage pneumatics (i.e., a spool valve or pneumatic relay), and a valve travel feedback sensor. The I/P converter is connected to a supply pressure and delivers a desired control fluid pressure to a flexible diaphragm abutting the spool valve. The diaphragm controls the position of the spool valve to direct the control fluid toward a chamber of the actuator. Movement of the actuator causes a corresponding movement of the throttling element, thereby to control flow of its process fluid. The positioner further receives a reference signal, typically in the form of a command signal, from a process controller, compares the reference signal to valve travel feedback, and drive the I/P converter (and second stage pneumatics) to move the valve toward the reference signal.
With the growing use of processor-based control, the spool valves used in positioners have become heavily instrumented. When used with a piston actuator, for example, the spool valve will include an inlet port for receiving supply pressure, a first outlet port fluidly communicating with a first chamber of the actuator, and a second outlet port fluidly communicating with a second actuator chamber. Spool valves are known in which a pressure sensor is positioned at the inlet port, first outlet port, and second outlet port for providing feedback to the processor. In addition, conventional spool valves include a displacement sensor for detecting the position of the spool valve and providing a feedback signal to the processor.
Conventional positioners have components that are susceptible to various control fluid leaks or blockages that may degrade or disable operation of the control valve. The I/P converter, for example, includes an inlet having a sealed connection with the supply pressure. The I/P converter includes a restriction defining a primary orifice and a nozzle for directing control fluid toward a flapper. The I/P converter further includes a sealed outlet for directing control fluid to the spool valve. The I/P converter is often located at an industrial site where the surrounding air may be contaminated with oil, dissolved minerals, grit, and the like. Consequently, when such air is used as the control fluid, the contaminants may partially or completely plug the primary orifice or nozzle. In addition, the seals provided at the inlet and outlet of the I/P converter may fail. Such blockages or leaks may slowly degrade the performance of the control valve, resulting in inefficiencies, or may cause complete failure of the control valve. In either event, it is difficult to determine that the positioner is the cause of the fault, let alone to determine the specific location of the fault within the positioner.
Similarly, leaks may develop in the actuator housing or blockages may form in the connections between the spool valve and the actuator that may degrade control valve performance or cause failure. For example, a leak may form between the upper or lower actuator chamber and atmosphere, or a piston ring may fail causing leakage from one chamber to the other. In any of these circumstances, the processor must adjust its control signal for a given position of the throttling element. Leak detection is particularly important when the control medium is natural gas. Such leaks may develop over time and, in a noisy plant environment, may go unnoticed until the valve no longer operates.